From: Ralph Seguin 
To: atm{at}shore.net
Reply-To: Ralph Seguin 


I make precision ring, ball and combination spherometers, and drill bits
simply don't compare to the ease, accuracy and speed of using an indicator
for a direct reading.  It also saves wear and tear on back and eyes...

Having seen some salty olt-timers using the old micrometer thread point
spherometers and then watching their reaction to an indicator based
spherometer is pretty neat.
I am in the process of donating some really nice spherometers to local
clubs and ATM groups.

IMHO, if you are going to use objects under the ruler method, I would
recommend bringing some good calipers and a variety of coins with you.  A
variety of currency from different countries sometimes helps :-)

-Ralph



-+- Anthony Stillman  wrote:
>
> Off list, a salty machinist pointed out that the
> tolerances of drill bits
> can be quite lose.  Aspiring to be salty myself, I
> too measured a few bits.
> Finding errors of:
>
>  +.0013
>  +.0009
>  -.0010
>  -.0011
>
> Well, that's not good.  And, how much does it
> matter?
>
> The table I presented related drill bit number to
> focal ratio for various
> sized mirrors.  The caveat to the table was an
> assumed width of bevel on
> the mirror.  Change the bevel width, change the
> numbers.  So, with that
> grain of salt and the properly sized drill bit in
> place, the focal ratios
> could be known to better than 0.1, usually.
>
> Now the bits can't be trusted. But it's not that
> bad.  For all but f/long,
> one one thousandth of change in the bit diameter,
> changes the focal ratio
> by about 0.1
>
> Bevel width and bit diameter's are likely
> un-correlated.  So it's possible
> the induced errors might occasionally cancel.
>
> The table should now be taken with two grains of
> salt.
>
>
> Anthony
>
>
>
> manus manum lavet.
> One hand washes the other.
>
>
>
>
>


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